The international chemical weapons watchdog has confirmed the UK's analysis of the chemical used in the poisoning of Sergei and Yulia Skripal, as Russia continues to deny responsibility for the attack.

The Organisation for the Prohibition of Chemical Weapons (OPCW) said its inspectors had found the nerve agent used was of "high purity" with an "almost complete absence of impurities."

The UK said the finding backed its assessment that the toxin was produced in the kind of controlled scientific environment most likely to be found in a state laboratory.

The executive summary released by the OPCW does not directly name Novichok - the military grade nerve agent developed by Russia, which the UK has said was used to poison the Skripals - or identify its source.

However it states that its analysis of biomedical and environmental samples collected by its inspectors "confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury."

It said that the name and structure of the chemical were included in its full classified report made available to member state governments.

The findings were welcomed by UK Foreign Secretary Boris Johnson who said they backed Britain's assertion that only Russia could have carried out the attack in Salisbury last month.

"This is based on testing in four independent, highly reputable laboratories around the world. All returned the same conclusive results," Mr Johnson said in a statement.

"There can be no doubt what was used and there remains no alternative explanation about who was responsible - only Russia has the means, motive and record.

"We invited the OPCW to test these samples to ensure strict adherence to international chemical weapons protocols. We never doubted the analysis of our scientists at Porton Down."

He said the UK has called a meeting of the OPCW executive council in The Hague for Wednesday to discuss "next steps", adding; "The Kremlin must give answers."

However Georgy Kalamanov, Russia's deputy minister of industry and trade, told the Interfax news agency that it was impossible to pinpoint the agent's origin and reiterated Moscow's demand for a fresh investigation with Russian involvement.

Russian foreign ministry spokeswoman Maria Zakharova accused Britain of engaging in "information warfare" over the case.

"The UK authorities are disregarding the standards of international law, diplomatic rules and principles, and elementary human ethics," she said.

In its report, the OPCW said it had been able to collect blood samples from the Skripals and from Detective Sergeant Nick Bailey who was also hospitalised in the incident, as well as environmental samples from contaminated "hot spots" in the area.

It also received splits of samples taken by the British authorities which were analysed in OPCW designated laboratories for "comparative purposes."

"The results of analysis of biomedical samples conducted by OPCW designated laboratories demonstrate the exposure of the three hospitalised individuals to this toxic chemical," it said.

The release of the report came as Ms Skripal, 33, who is recovering in a secret location after being discharged from hospital, revealed she had rejected an offer of consular assistance from the Russian embassy.

"I am not yet strong enough to give a full interview to the media, as I one day hope to do," she said in a statement released through the Metropolitan Police in London yesterday.

"Until that time, I want to stress that no one speaks for me, or for my father, but ourselves."

The Russian embassy said it doubted the authenticity of the statement and suggested it had been crafted to support the UK's version of events.